1. Field of the Invention
The present invention relates to a continuous process of using hot acidic medium for hydrolysis and fractionation of biomass into its major components, in several stages. In the continuous process, a continual shrinking bed reactor may be employed so that, as the biomass is solubilized, the reactor volume per unit feed decreases so as to keep the liquid to solid ratio relatively constant.
The continuous process of using a hot acidic medium for fractionation of biomass components (e.g., hemicellulose and cellulose sugars, lignin, and extractives) provides high yields of sugars, e.g. xylose and glucose.
Utilization of the continual shrinking bed reactor in the fractionation of lignocellulosic biomass so that the liquid to solid ratio is kept relatively constant increases yields of the solubilized sugars and increases concentrations of the released sugars by minimizing the residence time of the liquor fraction in the reactor.
2. Description of the Prior Art
Lignocellulosic biomass which is available in abundance can be used as an inexpensive feed stock for production of renewable fuels and chemicals. Current processes for this conversion involve chemical and/or enzymatic treatment of the biomass to hydrolyze cellulose and hemicellulose into their respective sugars. Enzymatic processes require the use of expensive biocatalysts and have the added burden of transporting lignin-slurries through the entire operating train. Current chemical processes for conversion of lignocellulosic biomass either require expensive chemical recycle or because of the prolonged exposure of the released sugars to the hydrolysis conditions, result in sugar degradation to by products. Accordingly, current processes for producing sugars from lignocellulosic biomass are expensive processes and low cost production of renewable fuels and chemicals using these current processes are not realized.
Further, in current continuous processes for the production of sugars from starch or lignocellulosic biomass, the reactors for hydrolysis of the lignocellulosic feedstocks by acid catalysis to produce carbohydrates for chemicals or fuels use reactor dimensions based on the bulk packing density of the feed material, thereby limiting the yields of solubilized carbohydrates as a function of hydrolysis conditions, and the reactors are expensive due to being designed for the incoming feed, and thus underutilize the entire reactor volume.
U.S. Pat. No. 4,880,473 entails a process for treatment of hemicellulose and cellulose in two different configurations. Hemicellulose is treated with dilute acid in a conventional process. The cellulose is separated out from the "prehydrolyzate" and then subjected to pyrolysis at high temperatures. Further, the process step between the hemicellulose and cellulose reactions require a drying step with a subsequent pyrolysis high temperature step at 400-600.degree. C. for conversion of the cellulose to fermentable products.
U.S. Pat. No. 5,366,558 uses two "stages" to hydrolyze the hemicellulose sugars and the cellulosic sugars in a countercurrent process using a batch reactor, and results in poor yields of glucose and xylose using a mineral acid. Further, the process scheme is complicated and the economic potential on a large-scale to produce inexpensive sugars for fermentation is low.
U.S. Pat. No. 5,188,673 employs concentrated acid hydrolysis which has benefits of high conversions of biomass, but suffers from low product yields due to degradation and the requirement of acid recovery and recycle. Sulfuric acid concentrations used are 30-70 weight percent at temperatures less than 100.degree. C.
An organic solvent for pretreatment of biomass in a counter current process configuration, using a single reactor in which small particles of biomass are introduced from the top and the solvent is contacted in a counter-current fashion from the bottom of the reactor is disclosed in U.S. Pat. No. 4,941,944. The process uses high concentrations (about 80%) of the solvent with a small amount of acid, if needed. The use of a solvent in this process necessitates recovery schemes which are cost-prohibitive insofar as the economics of the process is concerned.
Specific hydrolysis of cellulose by mild treatment with acid followed by treatment with high-pressure steam is disclosed in U.S. Pat. No. 4,708,746; however, the use of high-pressure steam and related capital-intensive equipment does not result in complete hydrolysis.
Biomass hydrolysis of almost exclusively hemicellulose sugars is disclosed in U.S. Pat. No. 4,668,340, wherein acid is introduced countercurrent to the biomass and is removed from each stage to be fed to the next in its sequence. The objective in this patent is to minimize the hydrolysis of cellulose. The objective of this patent (which deals with pre-hydrolysis of a lignocellulosic feed is ultimately to produce a cellulosic pulp containing 94-97% of the feed alpha-cellulose).
Both U.S. Pat. Nos. 5,125,977 and 5,424,417 relate to "prehydrolysis" of lignocellulosic biomass to solubilize the hemicellulosic sugars with concomitant release of some soluble lignin, thereby rendering the remaining cellulose more readily digestible with enzymes or other chemical means--thus these patents disclose only prehydrolysis.
Austrian Patent No. 263,661 discloses dissolution of the three major components of biomass (lignin, hemicellulose and cellulose) in a flow thru reactor using hot compressed water at temperatures between 140-350.degree. C. No yields of the carbohydrate fractions are disclosed in which the carbohydrates are fractionated "cleanly".
U.S. Pat. Nos. 1,014,311; 1,023,257; 3,480,476; 4,728,367; 3,787,241; 4,706,903; 4,645,541; and 5,398,346 disclose various and sundry processes for converting starch or lignocellulosic biomass using an array of reactors; however, none of these patents acknowledge or address any benefits associated with keeping the solid to liquid ratio the same or constant as sugars are solubilized and conveyed out of the reaction zone.
Heretofore, there has not been described a process for complete fractionation of lignocellulosic biomass using a dilute acidic medium in a flow-thru process in which the solid to liquid ratio of the lignocellulosic biomass and hydrolysis liquor has been kept the same or constant as sugars and other biomass components are solubilized and conveyed out of the reaction zone.